Claims:We Claim:

1. A common rail fuel injection system (100) comprising :
- a common rail (108) to accumulate fuel under high pressure
- a pump (106) to deliver pressurized fuel to said common rail (108)
- a metering unit to regulate the fuel supplied to the common rail (108)
- a pressure sensor (113) to read pressure of the fuel in the common rail (108)
- an injector (114) to inject fuel in a cylinder of an engine
said common rail fuel injection system (100) characterized by said pressure sensor (113) whose maximum operating range D is selected in dependence upon the capacity of said pump (106), leakage flow of said injectors (114) and injection capacity of said injectors (114)

2. A common rail fuel injection system (100) according to claim 1 wherein maximum operating threshold C of said common rail (108) is fixed below maximum operating range D of said pressure sensor (113)
3. A common rail fuel injection system (100) according to claim 1 wherein said common rail fuel injection system (100) is operated in limp home mode when pressure in common rail is above a maximum operating threshold
4. A common rail fuel injection system (100) according to claim 1 wherein a timer is started when common rail fuel injection system (100) enters said limp home mode
5. A common rail fuel injection system (100) according to claim 1 wherein engine is shut down when said timer times out
6. A common rail fuel injection system (100) according to claim 1 wherein during limp home mode, the ECU (112) calculates energizing time of said injector (114) based on the pressure of the fuel in the said common rail (108).

7. An engine control unit (112) to control a common rail fuel injection system, said ECU (112) adapted to :

- detect a fault in a metering unit
- set a timer
- operate said common rail fuel injection system in limp home mode
- shut down the engine if said timer times out

8. A method to operate a common rail fuel injection system (100) in a limp home mode, said method comprising the steps:
- detecting a fault in a metering unit
- setting a timer
- operating said common rail fuel injection system in limp home mode
- shutting down the engine if said timer times out

, Description:Field of the invention
[001] This invention relates to the field of common rail fuel injection system for an internal combustion engine. The invention relates in specific to operating common rail fuel injection system during the limp home mode of the vehicle.

Background of the invention

[002] Typically whenever there is any malfunction in a pump or a metering valve in a common rail fuel injection system, the pressure in the common rail may exceed a pre-defined threshold. For this purpose a pressure relief valve which is provided in the common rail, opens. This is detected by an Engine Control Unit (ECU) and the engine is operated in limp home mode where engine has only limited speed and can travel only a limited distance.

[003] The prior art DE 60208499T2 discloses a method of controlling a fuel injection system for use in delivering fuel to an internal combustion engine. In particular, the invention relates to a method of controlling the fuel injection system to provide a limp home function in the system in case of occurrence of a fault condition.

Brief description of the accompanying drawing

[004] Different modes of the invention are disclosed in detail in the description and illustrated in the accompanying drawing:

[005] Fig. 1 illustrates a common rail fuel injection system

Fig. 2 illustrates a graph of pressure p Vs time t of the system

Detailed description of the embodiments

[006] FIG. 1 illustrates a typical common rail fuel injection system 100. The common rail fuel injection system 100 is just referred as system in this document. The system 100 comprises a fuel tank 102, a low pressure fuel pump 103, a fuel filter 104, a metering unit, a high pressure fuel pump 106, a common rail 108, a high pressure fuel path 110, an ECU 112, a pressure sensor 113 and a set of injectors 114. Only the important components of the system 100 are shown.

[007] The low pressure fuel pump may be an electric feed pump which may be located inside the fuel tank 102. The feed pump 103 can be a mechanical feed pump which is part of high pressure pump along with Metering Unit. The low pressure pump draws the fuel from the tank 100. The fuel then passes through a fuel filter 104 which removes any solid particles from the fuel. The fuel passes through the metering unit. The metering unit, which is associated with the high-pressure pump includes a regulating valve which is actuated electromagnetically. The metering unit is disposed in the high-pressure pump 106, and the outlet of the regulating valve discharges into the low-pressure region of the high-pressure pump 106.

[008] The metering unit may form part of the high pressure pump 106 and may be integrated in the housing of the high pressure pump106. In another embodiment the metering unit may be external to the high pressure pump106. The metering unit regulates the flow rate of the fuel supplied from the feed pump to regulate the fuel pressure as required to supply fuel to the engine which is not shown. The metering unit supplies fuel at regulated pressure to the inlet of the high pressure pump 106. The metering unit may be controlled by an ECU 112 depending upon the operating conditions of the engine. This is achieved by controlling a solenoid provided in the fuel metering unit, by the ECU 112.

[009] The fuel is pressurized by the pump 106 and delivered to the common rail 108 which stores the fuel. To the common rail 108 are connected the injectors which inject fuel into the cylinders of the engine. The pressure sensor 113 continuously monitors the pressure in the common rail and provides this information to ECU.

[010] In the prior arts, the common rail has a pre-defined threshold for operating. It may be about 1600 bar. In the prior arts, the pressure sensor 113 is selected in such a way that its maximum operating range is little above the pre-defined threshold for the common rail, typically 1800 bar is the maximum operating range of the pressure sensor. In such a scenario, in the prior arts, if the metering unit becomes faulty, the pressure may exceed beyond 1800 bars, to say about 2300 bars. The pressure sensor 113 reads only 1800 bars and the ECU 112 calculates the energizing time of injectors based on 1800 bar which is maximum rated value for the pressure sensor. Here the pressure reading is wrong and hence the injection quantity is wrong. If the wrong quantity of fuel is injected, it may lead to unsafe operation of the engine and the vehicle.

The invention proposes a system to overcome this problem. This is explained with respect to Fig. 2.

Shown in fig. 2 is a graph of pressure Vs time. The X axis represents time t and the Y axis represents pressure p of the fuel in the common rail.

[011] Assume that typical maximum operating pressure of the system 100 is fixed at 1600 bars for the common rail. This is shown as the part A on the graph. Based on capacity of the pump 106, leakage flow of the injectors 114, and injector injection quantity per unit time, the pressure sensor 113 is so selected that it withstands metering unit fault to a maximum pressure value. This may work to about say, 2300 bars depending upon capacity of the pump 106, leakage flow of the injectors 114, and injector injection quantity per unit time. This is represented as the line C in the fig. 2. Based on all these factors the pressure sensor 113 is selected for a maximum rating of about 2500 bars. This is shown as line D in fig. 2. If there is fault in metering unit, the ECU 112 detects it by identifying voltage and current readings of the metering unit. This is represented as point B on the graph. Once it is detected by the ECU 112 that the metering unit is faulty, the vehicle is put in limp home mode and a timer is set. The pressure reading from pressure sensor 113 is read by the ECU 112. Now as the pressure sensor 113 has higher operating range as shown by line D, the reading will be correct. The energizing time of the injector is calculated by the ECU 112, depending upon reading from pressure sensor 113. The ECU 112 operates the injectors to inject the fuel as per limp home mode requirement. Once the timer times out, the engine is shut down by the ECU 112.

The limp home mode is collectively associated to engine, vehicle and common rail fuel injection system and these are used interchangeably in this document.

[012] The limp home mode is the one where the quantity of fuel to be injected is predefined. In the limp home mode the sped of the engine is also limited to a pre-defined value. This will enable the vehicle to be driven to the nearest garage for repair.

[013] If the metering unit starts malfunctioning, the common rail 108 receives more fuel than what is required to maintain pressure below pre-defined threshold. In such scenario, the pressure exceeds the threshold and may lead to unsafe operation of the system 100 and the engine.

The invention proposes a method to safely operate the vehicle in the limp home mode.

[014] The invention also proposes an ECU 112 adapted to determine the failure of the metering unit. This may be achieved by detecting any abnormal voltage and current readings of the metering unit. After detecting the failure of the metering unit, the engine is operated in limp home mode for a pre-defined time after which the engine is shut down.

[015] The invention makes use of known components in the fuel injection system to operate the common rail. The method is simple and implemented by the ECU 112. The ECU 112 uses known input and output interfaces to receive the signals from various sensors like pressure sensor 113 and control various actuators like metering unit, injectors etc.

[016] The invention does not need any additional components like pressure regulating valve to activate the limp home mode. There is a safe operating range available between the maximum operating threshold of the common rail and the maximum operating range of the pressure sensor. This safe operating range is made use of when the metering unit goes faulty.

[017] The invention proposes a common rail fuel injection system 100 comprising : a common rail 108 to accumulate fuel under high pressure; a pump 106 to deliver pressurized fuel to said common rail 108; a metering unit to regulate the fuel supplied to the common rail 108; a pressure sensor 113 to read pressure value of the fuel in the common rail 108; an injector 114 to inject fuel in a cylinder of an engine; said common rail fuel injection system 100 characterized by said pressure sensor 113 whose maximum operating range is selected in dependence upon the capacity of said pump 106, leakage flow of said injectors 114 and injection capacity of said injectors 114